Because of a law known as "14 US Code Section 89" which authorizes the Coast Guard to board any vessel in US waters at any time, without the need for probable cause:
http://codes.lp.findlaw.com/uscode/14/I/5/89

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Because of a law known as "14 US Code Section 89" which authorizes the Coast Guard to board any vessel in US waters at any time, without the need for probable cause:
http://codes.lp.findlaw.com/uscode/14/I/5/89
